what is the maximum allowable bending stress formula for pipe? deformation needs to be elastic 1

[delagina]

from aisc 9th edition 0.6fy or 0.66fy. does asme have different formula?

I'm lifting a pipe and wants to know the maximum unsupported length and maximum cantilever length.
pipe is just self weight. i can calculate the maximum moment and section modulus of pipe.

I just need the capacity formula to compare it to. also deflection is not an issue during lifting as long as deflection is elastic.

 

[JStephen]

I don't know what the pipe codes allow, the pressure-vessel code is more conservative than the building codes, though.

 

[SlideRuleEra]

ASME B31.1 Power Piping, Paragraph 121.2 Allowable Stress Values, limits bending stress to 0.3f_y not to exceed 9500 psi. This is for an operating pipe line and is worthless information for the lifting purpose you describe.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[racookpe1978]

"Pipe" has rather loose tolerances compared to "round structural metal members" of the same size and thickness because "pipe" is not subject to catastrophic buckling under end loads and bending loads, but instead is "straightened out" and "rounded out" under its usual internal pressure loads. On the other hand, very very few structural members are subject to 1200 or 1000 degree heat either.

tolerances will work against you all of the wrong ways:
You need to do the math, but you must:
-use the maximum allowed wall thickness when figuring the weight per unit length. Gives you highest weight.
-use the minimum allowed wall thickness when figuring the buckling stress at the most critical position (the point of buckling starting) which gives you the minimum expected strength to resist the buckling
-use the minimum allowable metal strength

But try both the minimum and the maximum allowed diameter at the point of buckling: I'm not sure which is more likely to fail.

Also, assume a point load at the hook - as if all the weight were at the bottom of the pipe at single point. If you ALWAYS use a sling or a support to spread the load out below the hook, then you could assume a linear load the length of the lifting clamp.

I say buckling (not long radius bending) because your failure will happen when the pipe kinks, not when it sags. Or when the pipe sags so much you cannot lift the middle high enough to pick both ends up off the floor. (For example, in a 20 foot high building with a 16 foot crane height, you cannot lift a 120 foot long string of pipe high enough to get it out the door.)

 

[BUGGAR]

Buckling stress may be well below specified yield. In large O.D. pipe, buckling stress is generally related to d/t of the pipe.

 

[delagina]

for a 15' cantilever for 42" diameter, 0.75 thick pipe. (selfweight is 330lb/ft)
Moment max = 446 kip-in, Sx of pipe = 984.73 in3
stress = 0.45ksi

even at 0.3 fy = 0.3*35 = 10.5ksi

am I missing something here? since I'm neglecting deflection, 15' even 20' cantilever is more than ok

 

[BUGGAR]

So now he tells us the pipe details! Yes, you are missing something here!

 

[Mixtli]

Hi,

Bucking is more for compression, not flexure. Althogh there are flexure limits on AISC due to local buckling of flanges.
Not sure what it says about round shapes.and its local buckling of wall.

15' cantilever doesn't seem crazy to me for a 3.5' diameter pipe.

T

 

[azcats]

Check section F8 of the 13th Ed. specification.

Local Buckling in flexure for that pipe is not applicable as it is a compact section. In more slender members local buckling can start to control flexure strength.

I show the nominal flexural strength of that pipe to be 3722.6 kip-ft via formula F8-1 and an assumed Fy of 35 ksi.

That pipe can easily cantilever 15' under self weight. I've seen 80' long sections of pipe choked with two straps about 10' apart in the middle and the deflection is hardly perceptible.

You'll also find that the code tables using these formulae (like 3-13 for square tubes) reduce all the allowable stresses by 7% due to specification tolerance and mills that are really good at hitting the bottom end of the spec. Take that for what it's worth in your application.

 

[racookpe1978]

Weel, yes. Buckling IS normally associated with long columns under axial loads. It is a classic definition.

But not in this case. If you pick up a very long hollow member by the middle, the very long flexible member (the pipe obviously) WILL bend down on both ends with the highest stress where the crane hook (chain or cable) is wrapped around the pipe. the middle of the pipe goes up as the crane lifts, the ends sag down (and slide towards the middle) until the the whole pipe is lifted off of the ground.

But if the "pipe" (or hollow tubing, or sheet metal gutter, or 1/4 tubing, or 40 foot length of poly) is too flimsy (has too small a S value) then the middle of the pipe buckles as the shape collapses. Zero axial load, but only bending load at the sharp-edged lifting point.

 

[SAIL3]

no problem...just make sure the load is sufficiently spread out @ the pick-up point as Racookpe pointed out...

 

[SAIL3]

to add to above post...@ the pick-up point, the conc load would be 15x2x0.33 = 9.9 kips....a good reason to spread out this load @ the pick-up point....